[JonChesterfield]

I think this is a worthwhile direction.

For example, I'd like to program against a sequence abstraction. When sort is applied to it, I hope it's a vector. When slice or splice, I hope it's some sort of linked structure. Size is as cheap as empty for the vector but much more expensive for a linked list.

It should be possible to determine a reasonable data representation statically based on the operations and control flow graph, inserting conversions where the optimal choice is different.

The drawback of course is that people write different programs for different data structures. Knowing what things are cheap and what aren't guides the design. There's also a relinquishing of control implied by letting the compiler choose for you that people may dislike.

As an anecdote for the latter, clojure uses vectors for lambda arguments. I thought that was silly since it's a lisp that mostly works in terms of seq abstractions, why not have the compiler choose based on what you do with the sequence? The professional clojure devs I was talking to really didn't like that idea.

[samatman]

Clojure uses vector syntax for lambda arguments. `read` sees a vector. What comes out of eval is a lambda. Does a Vector get built in the process? You'd have to check, my bet would be that the argument list spends a little while as a Java Array, for performance reasons, but that a Clojure Vector is not actually constructed.